Mosquito Repellent

Mosquito repellents are substances that are designed to make surfaces
unpleasant or unattractive to mosquitos. They typically contain an active
ingredient that repels mosquitos as well as secondary ingredients, which
aid in delivery and cosmetic appeal. They are available in many forms,
from creams to lotions to oils, but are most often sold as aerosol
products.

History

Traditionally, various types of substances have been used to repel
mosquitos. These include such things as smoke, plant extracts, oils, tars,
and muds. As insect repellent technology became more sophisticated,
individual compounds were discovered and isolated. This allowed the
formulation of new, more efficient forms of mosquito repellents.

The first truly effective active ingredient used in mosquito repellents
was citronella oil. This material is an herbal extract derived from the
citronella plant, an Asian grass. While citronella had been used for
centuries for medicinal purposes, its repellence was only accidentally
discovered in 1901, when it was used as a hairdressing fragrance. Since
citronella oil is a fragrant material, it is thought that the chemical
terpenes of which it is composed are responsible for its repellent
activity. Citronella oil does repel mosquitos, but it has certain
characteristics which limit its effectiveness. For example, it is very
volatile and evaporates too quickly from surfaces to which it is applied.
Also, large amounts are needed to be effective.

The disadvantages of using citronella oil prompted researchers to study
alternative synthetic compounds. Many of the early attempts at creating
synthetic insect repellents were initiated by the United States military.
Out of this research came the discovery of the repellent dimethyl
phthalate in 1929. This material showed a good level of effectiveness
against certain insect species, but it was ineffective against others. Two
other materials were developed as insect repellents. Indalone was found to
repel insects in 1937, and Rutgers 612 (2-ethyl-1,3-hexane diol) was
synthesized soon after. Like dimethyl phthalate, these materials had
certain limitations which prevented their widespread use.

Since none of the available materials were ideal repellents, research into
new synthetic materials continued. In 1955, scientists synthesized DEET
(n-n-diethylnetatoluamide), currently the most widely used active
ingredient for mosquito repellents. After its discovery, repellent
manufacturers developed many different forms in which to deliver DEET,
such as creams, lotions, and aerosols.

Mode of Action

Most repellent chemicals work by interfering with the mosquito's
homing system. This homing system, located on the antennae, is made up of
a number of chemical receptors. Research has shown that these chemical
receptors are activated by lactic acid, which naturally evaporates from
the skin of warm-blooded animals. The mosquitos have the innate ability to
follow the lactic acid emissions to their source. However, when a
repellent ingredient such as DEET is applied to the skin, it also
evaporates. It is thought that the chemical inhibits the binding of the
lactic acid to the mosquito's
chemical receptors. This essentially "hides" the protected
person from the mosquito. Since the active ingredient must evaporate from
the surface to work, the repellent activity lasts for a limited time.

Raw Materials

The active ingredient in a mosquito repellent is primarily responsible for
its usefulness. For a material to be valuable as a mosquito repellent, it
must meet certain criteria. First, it must effectively discourage insect
attack on the treated area for many hours and on many different types of
surfaces. Second, it must work under a variety of different environmental
conditions. Next, it must not be toxic or cause irritation when applied to
human or animal skin. Additionally, it must be cosmetically acceptable,
having a pleasant odor, taste, and feel. It should also be harnless to
clothing. Finally, it should have a relatively low cost and be effective
against other common types of insects, such as flies.

While thousands of compounds have been studied for their use as insect
repellents, DEET (n, n-diethyl-m-toluamide) has been used more than any
other. DEET is the compound which results from a reaction of
m-
toluic acid with thionyl chloride followed by a reaction with diethyl
amine. This material is isolated and purified before it is supplied to
mosquito repellent manufacturers. Other repellent ingredients used include
citronella oil, dimethyl phthalate, lavender, lemon-grass oil, and
peppermint oil. It has been found that mixtures of various repellent
compounds often provide greater effectiveness than any one compound alone.
The active ingredients contained in the mosquito repellents generally make
up 5-30% of the final products.

The inert ingredients that are in a mosquito repellent depend on the form
that the product will take. Currently, mosquito repellents are sold as
aerosols, pumps, lotions, and oils. Mosquito repellents that are sold as
lotions or creams are essentially skin creams which have DEET added at a
certain level. They are primarily composed of water, surfactants, fatty
alcohol, fragrance, and other emollients. When applied to the skin, these
products have the dual benefit of repelling mosquitos and moisturizing
skin. These products are generally less effective than aerosol forms,
however, because they do not allow the active ingredients to evaporate as
easily.

Aerosols are the most common form for mosquito repellents. They are made
up of a few different types of ingredients, including a solvent, a
propellant, and miscellaneous ingredients. The solvent is usually an
organic alcohol such as ethanol or propanol, whose primary responsibility
is to dilute the active ingredient to an appropriate concentration. It
also aids in keeping all of the raw materials mixed, ensuring that the
product will remain effective even after long-term storage. The propellant
is a volatile compound which creates the pressure that causes the rest of
the product to be forced out of the container. Common propellants include
liquified hydrocarbon gases like propane, butane, or isobutane,
hydrofluorocarbons, and dimethyl ether. Other ingredients such as
fragrances and emollients are added to aerosol mosquito repellents to make
them more cosmetically appealing. Still other compounds are added to
prevent corrosion and other stability problems.

In addition to the ingredients, the packaging components are also an
important part of an aerosol mosquito repellent. The can is typically a
metal container made up of tin-plate steel. The coating of tin keeps the
steel from reacting with the ingredients used in the repellent
formulation. The valve is another key packaging component. It has the dual
task of sealing the pressurized contents in the can and controlling the
dispensing of these contents. Valves have three sections: a diptube, which
feeds the product from the can to the valve body; the valve body, which
mixes the product and propellant; and the actuator button, which when
pressed, allows the product to be released.

The Manufacturing
Process

The production of mosquito repellents can be broken down into two steps.
First a large batch of the repellent formulation is made, and then the
batch is filled into the packaging. Since aerosols are the most common
form of mosquito repellent, the following

description details their production. Other forms of repellents like
creams and lotions are produced in a similar way, except that the filling
process is less involved.

Compounding

1 The first step in the manufacturing process is compounding. In the
compounding area, raw materials are mixed together in large stainless
steel tanks. For an aerosol, the alcohol is pumped into the tank, and
the other materials, including DEET, fragrance, and emollients, are
manually poured in and allowed to mix. All of the ingredients except the
propellant are added at this phase of production. Since some of the
materials in this process are flammable, special precautions are taken
to prevent explosion, such as using spark-proof electrical outlets and
blast-proof walls.

2 When the batch is finished, a sample is sent to the quality control
lab and tested to make sure it meets the set standards for the product.
After passing these tests it is pumped to the filling lines to make the
finished product.

Filling

3 The filling line is a series of machines connected by a conveyor belt
system that combine all of the components to make the finished mosquito
repellent product. The first machine in the system feeds the empty cans
onto the conveyor line. This machine has a large hopper that is filled
with empty

cans which are physically manipulated until they are standing upright
and correctly oriented.

4 The metal cans are then automatically moved along the conveyor belt
and cleaned with a jet of compressed air to remove any dust and debris.
They next travel to the filling carousel. The filling carousel is made
up of a series of piston filling heads that are calibrated to deliver
exactly the correct amount of product into the cans. As the cans move
through this section of the filling line, they are filled with product.

5 The next step in the filling process involves topping the cans with a
valve, adding the propellant, and pressurizing the cans. The valve is
put on by the valve inserter machine. Much like the bin that holds the
empty cans, the valves are also put in a hopper and then correctly
sorted and aligned. As the cans pass by, the valves are put on. These
valves are then tightly affixed to the can by the valve crimping
machine. Depending on the type of filling technique, the propellant is
either injected through the valve at high pressure or injected into the
can before the valve is crimped.

6 After the cans are capped and filled, they are moved to a hot tank, a
long trough filled with hot water. Here the cans are checked for
escaping bubbles that would indicate a propellant leak. The high
temperature of the waterbath also raises the
internal pressure of the can, which is intended to cause any weak spots
in the can to fail. This is a crucial quality control step that prevents
damaged cans from being sold to the public.

7 When the cans exit the waterbath, they are dried by high-pressure air
jets. Other components are then added, such as the actuating button and
the overcap. Any needed labels or printing are also added at this point.

8 The finished cans are then moved to the boxing area, where they are
put into boxes, typically a dozen cans at a time. These boxes are then
stacked onto pallets and hauled away in large trucks to distributors.
High speed aerosol production lines like the one described can move at
speeds of about 200 cans per minute or more.

Quality Control

Quality control is an essential step in the production of mosquito
repellents. Tests are performed at various points in the manufacturing
process to ensure that the finished products are consistent from run to
run, remain effective over a long period of time, and are safe to use.

Before production begins, the incoming raw materials are checked to ensure
they meet the previously set specifications. Tests such as pH, specific
gravity, and moisture content are typically performed. Additionally, the
cans are inspected for dents, corrosion, and other weaknesses. During
manufacture, samples of the repellent are taken during different points
along the filling line, and the characteristics or the product are tested.
Some of the monitored parameters include the level of active ingredient,
pressure, spray rate, and spray pattern. Other testing is conducted to
ensure that the cans evacuate properly. Also, long-term stability studies
may be done to establish that the cans do not show undue signs of
corrosion.

The Future

The use of many available mosquito repellents is not without its
drawbacks. Products that use DEET or citronella oil as the primary active
ingredients have been reported to causes rashes in some people. There have
even been cases in which children who used DEET products have become very
ill. For this reason, research has focused on finding new types of
repellents and methods for improving the safety of the ones that are
currently available. One recent advance in repellent technology is the use
of chemicals to "encapsulate" DEET. It is thought that this
product form will protect the user from the harmful effects of DEET while
still maintaining its repellent activity. More investigation will have to
be completed before this is verified.